The present invention relates generally to photolithography methods and systems, and more specifically to improved techniques for determining phase errors associated with phase shift masks.
Integrated circuits are made by lithographic processes, which use lithographic masks and an associated light or radiation source to project a circuit image onto a wafer. As should be appreciated, the kind of lithography generally depends on the wavelength of the radiation used to expose the resist (e.g., UV radiation, x ray radiation, e beam, ion beam and the like). Referring to FIG. 1, a simplified diagram of a lithography system 2 is shown. By way of example, the lithography system may correspond to a stepper or scanning system. The lithography system 2 typically includes a light or radiation source 3 and a first set of optics 4 that illuminate a reticle or mask 5 having a circuit pattern 6 disposed thereon. The lithography system 2 also includes a second set of optics 7 that pick up the transmitted light or radiation and focuses (or images) it onto a surface 9 of a semiconductor wafer 8 thus writing the pattern of the mask 5 onto the surface 9 of the semiconductor wafer 8. In most cases, the semiconductor wafer 8 includes a layer of photoresist that when exposed to the patterned light or radiation forms the pattern of the mask onto the wafer. The reticles or masks come in various forms. However, they typically include opaque portions (e.g., chrome lines) and/or transmissive portions (e.g., quartz). The opaque portions block the light from passing through the mask 5 while the transmissive portions allow the light to pass therethrough.
Alternating Phase Shift Masks (AltPSM), which include opaque portions separated by alternating phase shifted transmissive portions, have become an important and enabling technology currently reaching maturity within the lithography environment. This technology, although initially proposed simultaneously in Japan and the United States as long as 20 years ago, is starting to become a mainstream technique for achieving smaller k1 values in the lithography of critical process layers, specifically the poly to gate layer. Alternating Phase Shift Masks are used at this process layer in particular since the requirement for the smallest possible pitch is crucial in terms of packing density. Alternating Phase Shift Masks achieve a reduction in minimum pitch by disposing phase shifted transmissive portions, in which the optical phase alternates by 180 degrees, in between the opaque portions. In most cases, the AltPSM includes chrome lines (opaque portion), which are separated by different phase shifted portions of a quartz substrate (transmissive portions). The different phase shifted portions are typically formed by etching the quartz substrate. The different phase shifted portions may also be altered by the addition of an attenuating phase shifter (e.g., attenuated phase shift reticles).
FIG. 2 shows an alternating phase shift pattern 10 of a phase shift mask 12, an amplitude profile 14 of the light passing through the alternating phase shift pattern 10, and a resulting pattern 16 formed on a wafer 18. As shown, the phase shift mask 12 includes a substrate 22 formed from quartz, a plurality of chrome lines 24 and alternating phase shift zones 26 and 28 disposed between the chrome lines 24. The phase shift zones 26 correspond to the exposed surface of the quartz substrate 22 and the second phase shift zones 28 correspond to portions of the quartz substrate 22 that have been etched. The phases of the phase shift zones 26 and 28 are opposite one another (e.g., 180 degrees). As should be appreciated, as light traverses the quartz substrate 22, there is a phase shift that is induced in the light that traverses the etched quartz compared to the light that traverses the unetched quartz. The opposite phase creates a situation where the light intensity has to go through zero. These zero points correspond to where the light destructively interferes thus forming a dark line between the two phase shifted areas 26 and 28. The dark lines do not expose the photoresist 30 disposed on the wafer 18 and thus a resulting pattern 16 is formed on the wafer 18.
Unfortunately, however, this method introduces a number of new challenges. One of the challenges is achieving consistent and accurate phase differences between adjacent transmissive portions in the Alternating Phase Shift Masks. This is critical since phase errors associated with phase difference can introduce additional pattern placement errors of the lines and spaces in the subsequent lithography process thereby making overlay control difficult. As should be appreciated, overlay generally refers to the determination of how accurately a first circuit pattern aligns with respect to a second circuit pattern. Overlay errors are generally determined by measuring the relative shift between first and second overlay targets with overlay metrology tools. This new kind of pattern placement error cannot be measured using standard overlay metrology targets. Phase errors, however, can be characterized by detailed measurements of the Alternating Phase Shift Masks themselves on a dedicated Alternating Phase Shift Masks inspection tool. Typically such Alternating Phase Shift Masks inspection tools are available only at the point of reticle manufacture. That is, they are not located within the production environment. By way of example, one such phase shift inspection tool is manufactured by Lasertec of Japan.
In view of the foregoing, there is a desire for improved techniques for determining phase error associated with phase shift masks, and further improved techniques for determining phase error associated with phase shift masks at a point during wafer processing.